1. Field of the Invention
The present invention relates to a light emitting apparatus and, more specifically, to a light emitting apparatus generating white light by mixing of light of a plurality of oscillation wavelengths.
2. Description of the Background Art
Studies have been made to develop a white light source of high quality using a light emitting diode (also referred to as an LED). The white light source using a light emitting diode is utilized in, for example, a backlight of a liquid crystal display device, a luminaire or an image reading apparatus.
Methods of generating a white light source with a light emitting diode are broadly divided into a method using a fluorescent material and a method using a plurality of oscillation wavelengths. In the method using a fluorescent material, a fluorescent material is used for converting light ranging from ultraviolet to blue emitted from the light emitting diode into colors such as yellow, green and red to generate a white color. In the method using a plurality of oscillation wavelengths, a plurality of light emitting diodes having two, three or more different oscillation wavelengths are turned on to generate a white color.
With either method, however, it is difficult to actually obtain desired chromaticity and intensity of emitted light.
In the former method using the fluorescent material, brightness of the ultraviolet-to-blue light emitting diode varies and chromaticity largely differs due to variation in application of the fluorescent material. Furthermore, once the white light source using the fluorescent material is manufactured as a product, adjustment of the chromaticity becomes substantially impossible.
The latter method using a plurality of oscillation wavelengths is disclosed, for example, in the following documents.
Japanese Patent Laying-Open No. 2001-272938 discloses a tone adjustment circuit which can correct a variation in tone of each LED by controlling a forward current flowing through a monochromatic LED to control the tone of emitted light, and discloses an LED display device including the circuit.
Japanese Patent Laying-Open No. 2002-324685 discloses a luminaire which can adjust chromaticity and brightness of light for illumination by controlling a level of a current supplied to a light emitting diode and a ratio between ON and OFF times.
Japanese Patent Laying-Open No. 2004-086081 discloses a color display device including a time memory circuit for storing a time of light emission of a plurality of light emitting diodes and a control unit for varying the time of light emission of the light emitting diodes based on stored information of the time memory circuit, in which a white balance of light obtained by light emission of a plurality of light emitting devices is adjusted by rewriting the stored information of the time memory circuit.
Japanese Patent National Publication No. 2002-533870 discloses an LED luminaire emitting white light, which includes a plurality of LEDs for each of red, green and blue colors respectively having separate power supplies, and a photodiode formed to measure light outputs of all of the LEDs.
A conventional tone adjustment circuit disclosed in Japanese Patent Laying-Open No. 2001-272938 will now be described in detail referring to the drawing.
FIG. 12 is a circuit diagram indicating a specific circuit construction of a conventional tone adjustment circuit 100.
Referring to FIG. 12, the conventional tone adjustment circuit 100 includes a light emitting diode 101 using InGaN (indium gallium nitride), a variable resistance 102, a transistor 103, and a pulse width modulation circuit (hereafter referred to as a PWM circuit) 104. Light emitting diode 101, variable resistance 102 and transistor 103 are connected in series between a power supply node Vcc and a ground node. PWM circuit 104 is connected to a base of transistor 103 and applies a driving voltage having a modulated pulse width via the base.
The conventional tone adjustment circuit 100 adjusts a value of a forward current flowing through light emitting diode 101 by adjusting a resistance value of variable resistance 102. With this, a tone of light emitting diode 101 can be adjusted. In addition, the conventional tone adjustment circuit 100 changes a pulse width (a time width) of the forward current with PWM circuit 104 to adjust a duty ratio. With this, luminous intensity of light emitting diode 101 can be adjusted.
As described above, the conventional tone adjustment circuit or the like disclosed in each of the aforementioned documents adjusts an amount of a flowing current for each light emitting device to generate desired chromaticity, and changes a pulse width or a duty ratio of a driving voltage to adjust intensity of emitted light. This is because, in a common light emitting diode, an oscillation wavelength varies when an amount of a current flowing therethrough changes, and consequently chromaticity changes. Therefore, once the chromaticity is determined, generally a current value will not be changed and, alternatively, a lighting time is changed to vary brightness of the light emitting device.
Adjustment of chromaticity alone without that of intensity of emitted light can be performed by changing a ratio of a lighting time of a light emitting diode of each oscillation wavelength. Japanese Patent Laying-Open No. 2001-209049 discloses a luminaire and a liquid crystal display device, each of which has a light emitting diode separately provided for correcting chromaticity, in which intensity of light emitted from the diode is changed with a current value to adjust the chromaticity.
Furthermore, a light emitting diode is devised which has light emitting regions for different wavelengths in one device and mixes oscillation wavelengths thereof to emit white light with one device. Japanese Patent Laying-Open No. 2002-368268 discloses a compound semiconductor light emitting device which has a multiple quantum barrier layer for separating at least two well layers having different light wavelengths to enable emission of a plurality of different light wavelengths within one device to implement emission of white light.
When a plurality of white light sources are aligned to be used as a backlight of a liquid crystal display device or a light source for illumination, even a small difference in chromaticity among the white light sources will give an unnatural impression because human eyes feel a difference in colors relatively large by comparison. Therefore, chromaticity of the white light sources have to be made as even as possible.
In the conventional method of generating white light source using a plurality of oscillation wavelengths, since a current flowing through each light emitting device is controlled independently, independent adjustment means of the same number as that of light emitting devices or oscillation wavelengths are required. Therefore, a whole driving circuit becomes large and complicated, resulting in increased cost. In addition, since adjustment of chromaticity is required for each oscillation wavelength, an adjustment operation becomes complicated.